THE Queen's racehorse Estimate which won at last year's prestigious Royal Ascot meet has failed a drugs test, Buckingham Palace announced tonight.

The Queen's racehorse Estimate has failed a drugs test after testing positive for morphine[PA]

The 2013 Gold Cup winner, which came second in the same race this year, tested positive for prohibited substance and opioid morphine.

The palace said early indications suggests the positive drugs test was the result of consumption of a contaminated feed product.

Her Majesty, 88, who cheered the horse on to success at Royal Ascot last year has been informed of the news.

It was delivered to the Queen as her and her family celebrated grandson Prince George's first birthday.

The Queen cheering on Estimate in 2013 as it won the Gold Cup at Royal Ascot [PA]

Initial indications are that the positive test resulted from the consumption of a contaminated feed product

John Warren - the Queen's Bloodstock and Racing Advisor

A statement by John Warren, the Queen's Bloodstock and Racing Advisor, said the British Horseracing Authority (BHA) told the Palace five horses from various trainers tested positive for morphine.

He said: "I can confirm that one of those horses was Estimate, the five year-old filly trained by Sir Michael Stoute and owned by The Queen. Initial indications are that the positive test resulted from the consumption of a contaminated feed product.

"Sir Michael is working closely with the feed company involved to discover how the product may have become contaminated prior to delivery to his stables.

Estimate, ridden by Ryan Moore, at the prestigious Royal Ascot race this year [PA]

"As the BHA investigates this matter, including potential links between the different cases, Sir Michael continues to offer his full co-operation.

"There will be no further comment until the BHA announces its considered findings. Her Majesty has been informed of the situation."

Estimate made sporting history for the Queen last year when it became the first reigning monarch to win Royal Ascot's Gold Cup.

The Queen clapped her hands and had a beaming smile on her face when Estimate finished first in the race last June.

Her family, including Princesses Beatrice and Eugenie, jumped for joy behind Her Majesty in the Royal Box.

At the time Sir Michael said the Queen was "thrilled" with her horse's victory.

He said: "It's a special thrill to win this race for the Queen and it will have given her enormous pleasure - she really loves this game and it's a great recreation for her.

"She told me it was a very big thrill and thanked everybody involved."